So how is that any different than any other job? You are correct the kids will be able to tell you who the good teachers are and the poor ones. We as a staff know the same thing.
But how do you prove it for merit based raises? Are you going to listen to parents, maybe they have a grudge against a teacher, hell I have personally witness fine teachers pushed out because they pissed off a parent or administrator.
Like I said, any district that continues to employee poor teachers, that falls on administration, they can get rid of the them if they want too.
I remember teaching with a science teaching years ago, many parents and kids thought he was the biggest prick on the staff, if many had the chance, they would have forced him out. I thought the guy was a jerk at first myself, the kids literally wrote the book in their notes. What I noticed later on, was the kids that went to college, when they came back, they always wanted to thank him, on how well they learned science and how he had prepared them for college.
So one persons great teacher maybe someone else's worst nightmare. Like I said earlier, teaching is just like any other profession, a few great teachers in any school, a few horrible teachers and many are just average or a little above. Why should teachers be any different than any other field of work. We sure are not being paid like many fields, with the same amount of education as we have. Its just easy to pick on education, and tell all of us, we need to do a better job.